Stress fibers (actin filament bundles which are associated with other contractile proteins and proteins believed to be important for the control of cellular contractile activities) are well-developed in the cytoplasm of most tissue culture cells. The majority of cells in the body, however, do not have stress fibers. Among the mammalian tissue cells studied so far, only endothelial cells of the cardiovascular system contain stress fibers. Studies proposed in this application are aimed at elucidating the biological function(s) of stress fibers in vascular endothelial cells. The stress fiber of endothelial cells will be characterized by electron microscopy in order to obtain its detailed ultrastructural features, and by immunofluorescence microscopy in order to determine its macromolecular composition. Since in situ endothelial cells are difficult to manipulate experimentally and since cultured vascular endothelial cells are not physiologically identical to those of the in situ endothelium short-term organ cultures of the rat arterial wall will be used to study the stress fibers' function in situ. Stress fibers in tissue culture cells appear to play a role in cell adhesion, thus their involvement in cell adhesion will be investigated in situ. Since cell adhesion results from the interaction between the cell membrane, to which the stress fiber is anchored, and the extracellular matrix, attempts will be made to obtain evidence for a specific interaction between the stress fiber and the extracellular matrix. The long-term objective of this project is to understand how vascular endothelial cells are anchored to the basement membrane. Endothelial cells aer exposed constantly to fluid shear stress, thus tight adhesion to the basement membrane must be a major concern for this cell type. If endothelial cells lose their adhesion, serious pathological conditions such as atherosclerosis result. The investigation on the mechanism of the tight adhesion between endothelial cells and the basement membrane is an important first step toward finding a cure for these diseases.